The invention concerns a process and a device for polarizing at least one part of a cylinder-shaped ferroelectric material.
Most often, besides ceramics, the ferroelectric materials that are polarized are VDPF vinylidene polyfluoride or the copolymer VDPF-TFRE (trifluorethylene). More generally this involves polymers or copolymers.
After having been polarized, these materials have residual polarization and, as a result, have piezoelectric or pyroelectric properties allowing them to be used more particularly in sensors.
Processes and devices are already known for polarizing sheets of ferroelectric material, described by the applicant in the documents FR-2 538 157, FR-2 700 219, FR-2 700 720 and FR 2 270 863. These processes include the following steps:
an external electrode is placed on each face of the sheet of ferroelectric material to be polarized, so that the area to be polarized occupies the interval between these two electrodes;
this unit is highly compressed using a press;
high voltage is applied to the material, according to a predetermined cycle, using electrodes; polarization is then accomplished in the area of coverage between the two electrodes.
Compressing the sheet-electrode unit makes it possible to limit the possible local variations in volume due to the high values of the electrical field during the polarization phase, these variations in volume being likely to cause dielectric breakdowns or to affect the reproducibility of the features of the sensors. In practice, the pressure that can thus be exerted on the sheet of ferroelectric material is around 200 bars.
These processes allow one to obtain a known, predetermined level of polarization and to guarantee homogeneous and reproducible polarization through compression of the sheet-electrode unit. Furthermore, piezoelectric performance levels are much higher than with other processes such as the Corona process, for example.
However, these processes do not allow polarization of ferroelectric materials in non-pellicular form, for example, cylinders. In effect, these processes involve placing the materials undo flat table presses at pressures that can reach several hundreds of bars. Applying these processes to cylindrical-shape components would therefore result in the distortion and crushing of these components. Furthermore, in this way, the polarization process is not accomplished correctly.
The invention seeks to mitigate or eliminate the limitations of polarization processes known in the state of the art by proposing a solution that makes it possible to apply the polarization process described in the documents cited above to components made of a ferroelectric material with a cylindrical shape.
According to a first aspect, the invention regards a process for polarizing cylinder-shaped ferroelectric material, an internal conductor being placed inside and in the axis of said cylinder.
Since a cylinder possesses rotational symmetry, it is not necessary to place it under stress during the polarization phase to obtain homogeneous and reproducible polarization.
The process according to the invention includes the following steps:
at least one part of said cylinder is placed in a conductive liquid, said cylinder being free of any stress other than the pressure exerted by the conductive liquid;
a low frequency, AC voltage is applied between an electrode in contact with the conductive liquid and the internal conductor positioned inside and in the axis of the cylinder, which is increased progressively in amplitude until a value such that the electrical field thus created has an amplitude greater than the coercive field of polarization Ec of said material.
This process can be used more particularly to polarize flexible or stiff ferroelectric cables. After the polarization step, these cables can be covered with a braiding or conductive paint.
This process can also be used to polarize cylinders of ferroelectric polymers or copolymers.
According to one mode of embodiment, several parts of the same very long cylinder can be polarized successively: once the desired polarization is achieved for a first part of the cylinder, said cylinder is moved by pulling on internal conductor in order to remove said first part of the cylinder from the conductive liquid and to immerse a second part of the cylinder in order to polarize said second part.
According to a variant of this mode of embodiment, successive parts or the ferroelectric material in cylinder form an be alternately polarized positively and negatively.
According to another variant, only certain parts of the cylinder of ferroelectric material can be polarized.
According to a second aspect, the invention concerns a device for implementing this process, said device comprising a vessel containing a conductive liquid, an electrode in contact with the conductive liquid, means for inserting the cylinder-shaped ferroelectric material into the conductive liquid and means for applying low frequency AC voltage whose amplitude is increased progressively to a value such that the electrical field thus created has a higher amplitude than the coercive field of polarization Ec of said material between the electrode in contact with the conductive liquid and the internal conductor positioned in the axis of the cylinder.
According to one embodiment, the device also comprises means for rolling up the cylinder of ferroelectric material so that one can, for example, polarize long lengths of flexible ferroelectric cable made of a ferroelectric material in whose axis an internal conductor is positioned.
The process according to the invention may be used, among other uses, to produce an acoustical antenna or a polarized component for a piezoelectric sensor.
As when it is applied to sheets of ferroelectric material, polarization is controlled in level and may be accelerated in time.